A light-emitting diode (LED) that has newly attracted attention as a next-generation light source has merits such as high luminescence efficiency, high responsiveness, long lifespan, and miniaturization compared to an existing incandescent lamp, halogen lamp, and fluorescent lamp, and also has excellent characteristics as an environment-friendly light source that does not use mercury unlike the fluorescent lamp. Accordingly, the light-emitting diode has been widely used in a very wide industry field of signals, signs, displays, communications, mobile terminals, vehicles, and general illuminations. Especially, a white light-emitting diode based on such a light-emitting diode has been used for a back light unit (BLU) in LCD TV or a notebook computer, and a head lamp of a vehicle, and has been expected to continue high speed growth in an illumination market due to a cost reduction of the general illumination and an execution of a regulation policy for the incandescent lamp.
As a general method of realizing a white light-emitting diode, there are a method of using a combination of light-emitting diode chips that emit light rays with different monochrome wavelengths, and a method of using a combination of a light-emitting diode chip and a light-emitting material having a single component or multiple components. When the white light-emitting diode is realized by the combination of the plurality of light-emitting diode chips, since outputs of the chips are changed due to non-uniform in operation voltages applied to the chips and an ambient temperature, it is difficult to realize white light with high color reproducibility and high color purity. Accordingly, there has been generally used a method of manufacturing a white light-emitting diode by applying a light-emitting material and a sealing member made of a polymer material on a light-emitting diode chip having a monochrome wavelength of near ultraviolet light or blue light. In order to realize white light with high purity, a combination of the light-emitting diode chip and a single light-emitting material or a plurality of light-emitting materials having an emission wavelength of red, green, blue and yellow is used. That is, the light-emitting material in the white light-emitting diode serves to realize white and emission color of the light-emitting diode chip which is not absorbed by the light-emitting material by absorbing blue light (or near ultraviolet light) generated from the light-emitting diode chip, and converting the absorbed blue light into red, green, blue, or yellow light with a unique long wavelength of the light-emitting material.
The entire luminescence efficiency of a white light-emitting diode is a very important factor representing performance of the light-emitting diode, and in order to realize a white light-emitting diode having high luminance at a low power, it is necessary to increase light conversion efficiency of a light-emitting material. Furthermore, two or more types of light-emitting materials are needed to realize white light having high color purity. That is, an absorption wavelength of the light-emitting material needs to be appropriately overlapped with an emission wavelength of the light-emitting diode chip, and an emission wavelength thereof needs to be formed in a visible light range of a longer wavelength in order to realize white light. Moreover, the light-emitting material having high internal quantum yield is preferably used. Unfortunately, the absorption and emission characteristics of such light-emitting material are unique characteristics determined in a step of synthesizing or manufacturing the light-emitting material, and there are considerable limitations in controlling the absorption and emission wavelengths and manufacturing the light-emitting material having high quantum yield.
In order to solve these limitations, localized surface plasmon resonance (LSPR) of metal nanoparticles may be used. The localized surface plasmon resonance means a strong interaction between metal nanoparticles and light. When light (hv) is incident onto metal nanoparticles or nanostructures, surface free electrons of the metal nanoparticles are allowed to collectively oscillate along an electric field of the incident light to form a surface plasmon, and a very strong local electric field is formed around the metal nanoparticles. In this case, when the light-emitting material is located adjacent to the metal nanoparticles, since light absorption is increased due to the strong electric field locally formed around the metal nanoparticles, excitation enhancement (Eex) can be exhibited. As a result, an increase in luminous intensity of the light-emitting material can be expected. In addition, an emission enhancement (Eem) causing an increase in the unique quantum yield of the light-emitting material due to a mutual attraction between the excited light-emitting material and the surface plasmon can be expected. In this case, when the quantum yield is represented by a radiative decay rate (γrad) and a non-radiative decay rate (γnon-rad) in Equation 1, for a fluorescence material located around the metal nanoparticles, since the entire radiative decay rate (γrad+γM-rad) becomes considerably higher than the non-radiative decay rate (γrad+γM-rad>>γnon-rad) due to a metal-induced radiative decay rate (γM-rad) induced by the surface plasmon of the metal nanoparticles, the quantum yield is increased (see Chemical Reviews, 2011, 111, 3888; Nature Materials, 2010, 9, 193; Analyst, 2008, 133, 1308).
                              Q          =                                    γ              rad                                                      γ                rad                            +                              γ                                  non                  ⁢                                      -                                    ⁢                  rad                                                                    ,                                  ⁢                                  ⁢                              Q            Metal                    =                                                    γ                rad                            +                              γ                                  M                  ⁢                                      -                                    ⁢                  rad                                                                                    γ                rad                            +                              γ                                  M                  ⁢                                      -                                    ⁢                  rad                                            +                              γ                                  non                  ⁢                                      -                                    ⁢                  rad                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
That is, the total emission enhancement (Etotal) of the light-emitting material due to the surface plasmon of the metal nanoparticles is the product of the excitation enhancement Eex and the emission enhancement (Eem), and may be expressed as Equation 2 below.Etotal=Eex×Eem  [Equation 2]
Accordingly, in order to maximize luminous intensity enhancement of the light-emitting material due to the surface plasmon of the metal nanoparticles, it is important to simultaneously exhibit the excitation enhancement (Eex) and the emission enhancement (Eem). In order to control the excitation and emission enhancements, it is important to allow the absorption and emission wavelengths of the light-emitting material to be effectively overlapped with the surface plasmon bands of the metal nanoparticles (see Nano Letters, 2007, 7, 690; Applied Physics Letters, 2008, 93, 53106). For example, when the absorption wavelength of the light-emitting material is overlapped with the plasmon band, since the light absorption is increased, the excitation enhancement (Eex) of the light-emitting material can be expected. Meanwhile, when the emission wavelength of the light-emitting material is overlapped with the plasmon band, since the radiative decay rate is increased due to coupling of the excited light-emitting material and the surface plasmon, the emission enhancement (Eem) causing an increase in the quantum yield can be expected. Therefore, when the absorption and emission spectra of the light-emitting material are allowed to be appropriately overlapped with the surface plasmon bands of the metal nanoparticles, since the excitation enhancement and the emission enhancement of the light-emitting material can be simultaneously exhibited, it is possible to maximize the luminous intensity enhancement.
In recent years, there has reported a technology of realizing a light-emitting diode with an increased light conversion efficiency and a high luminance at low power by exhibiting the excitation enhancement and the emission enhancement of the light-emitting material in a light-emitting diode that realizes white light by combination of a light-emitting material and a light-emitting diode chip having a wavelength of near ultraviolet light or blue light by using a principle of the luminous intensity enhancement due to the surface plasmon of the metal nanoparticles (Korean Patent Registration Nos. 10-0659900, 10-0966373 and 10-1062789).
Disadvantageously, in the aforementioned patent documents, metal nanoparticles which are synthesized in a solution process by a bottom-up method or nanostructures having a single surface plasmon band, which are arranged on a substrate by etching a metal thin film by a top-down method, are typically used. In this case, there are considerable limitations in maximizing the luminous intensity enhancement by simultaneously exhibiting the excitation enhancement and the emission enhancement of the light-emitting material. For example, in the white light-emitting diode using a light-emitting diode chip having near ultraviolet or blue light wavelength and a yellow light-emitting material such as yttrium aluminum garnet (YAG), when spherical silver nanoparticles are used, since a surface plasmon band is generally formed around a wavelength of 400 nm to 500 nm in the spherical silver nanoparticles, the near ultraviolet or blue light wavelength of the light-emitting diode chip and an absorption wavelength of the yellow light-emitting material are effectively overlapped with each other, and thus, since absorption of the yellow light-emitting material is increased, it is possible to expect excitation enhancement efficiency. However, since an emission wavelength of the yellow light-emitting material and the surface plasmon band of the silver nanoparticles are not effectively overlapped with each other, it is difficult to expect the emission enhancement causing an increase in the internal quantum yield. Thus, it is difficult to maximize the luminous intensity enhancement.
Meanwhile, in the configuration of the same white light-emitting diode, when gold nanoparticles are used, since the surface plasmon band is formed in a wavelength of 500 nm to 600 nm, the emission wavelength of the yellow light-emitting material and the surface plasmon band of the gold nanoparticles can be effectively overlapped with each other. Thus, an emission enhancement causing an increase in internal quantum yield can be exhibited. However, since the surface plasmon band of the gold nanoparticles is not overlapped with the near ultraviolet light or blue light wavelength of the light-emitting diode chip, it is difficult to expect excitation enhancement due to an absorption increase of the light-emitting material.
When plurality (for example, two or more) types of light-emitting materials having different emission wavelengths such as blue, green, red and yellow are introduced to realize white light with high color purity, if the metal nanoparticles or nanostructures that form a single surface plasmon band are used, it is difficult to simultaneously exhibit emission enhancements of different types of light-emitting materials. For this reason, there are considerable limitations in realizing the light conversion light-emitting device with high luminance and high color purity.